摘要:

The scanning transmission electron microscope constructed at Johns Hopkins follows the general layout of the first instrument at the University of Chicago. It is currently operating at 50 kV with a resolution of about 3 A˚. Its detector scheme consists of scintillation crystals coupled to photomultipliers in such a way as to eliminate introduction of unnecessary statistical noise. A unique alignment scheme utilizes the spherical aberration of the objective lens.

ISSN:0034-6748    

DOI:10.1063/1.1135840

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A radiation source has been developed and implemented from Cˇerenkov emission that is intended to provide an intense continuum from the infrared to 600 A˚. Parasitic use of the primary electron beam at the Stanford Linear Accelerator Center (SLAC) together with a novel optical geometry for light collection can give a focused and tunable ultraviolet beam with 104kW/m2⋅sr brightness, 10−2spectral purity, and with the pulsed, 5 ps time structure of the SLAC electron beam. Measurements of emission characteristics in the visible part of the spectrum correlate closely with the predicted performance.

ISSN:0034-6748    

DOI:10.1063/1.1135841

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

An apparatus is decribed which has been used to determine the shear modulus of cylindrical specimens vibrating in the torsional mode. The specimens are maintained in self‐excited vibration by an eddy‐current driver, an eddy‐current pickup, and a feed back circuit consisting of a phase‐locked‐loop and an amplitude control module. The system has a frequency stability of 1 part in 106for high‐Qmaterials (a few thousand) in the audio range. In addition, it can automatically track frequency changes of a few percent with negligible error in phase or frequency.

ISSN:0034-6748    

DOI:10.1063/1.1135842

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

The design of a small compact pulsed blowdown apparatus coupled to a cold cathode electron gun is described. This system allows the excitation of a gas mixture in supersonic flow at temperatures of 80 and 120 K and density of 1 amagat with a maximum electron beam current density of 20 A cm−2at 300 kV and pulse length of 0.5 &mgr;s. With this facility it has been possible, using aerodynamic cooling, to achieve high‐power, long‐pulse ultraviolet laser operation in a high‐pressure active medium on XeF and Ne iiion transitions. A strong increase in fluorescence intensity was also obtained for numerous atomic lines and molecular bands of interest for the developement of high‐power, high‐efficiency, visible and ultraviolet lasers.

ISSN:0034-6748    

DOI:10.1063/1.1135843

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Dual sample cavities are described in which the standard sample sees both the primary modulation field and also an opposing secondary modulation field from a very small adjacent compensation coil. Both primary and secondary fields are at the same modulation frequency. The geometry permits solution of dual sample cavity design problems at Q‐band, where the microwave cavity is small relative to the volume of space occupied by the primary modulation field and at X‐band when magnets with a small volume of homogeneous field are employed.

ISSN:0034-6748    

DOI:10.1063/1.1135844

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A nuclear magnetic resonance (NMR) flowmeter has been developed for noninvasive measurement of upper extremity blood flow in humans. Flow through major arteries and veins is determined by placing the arm into a 12.5‐cm i.d. crossed‐coil detector structure, contained in a 0.075 T (750 G) steady detector field. Venous flow is measured by passage of a demagnetized bolus of blood, generated by periodic application of a radio‐frequency tagger field upstream from the detector. The self tag technique is applied to arterial flow. Detector output is a direct function of the pulsatile component of arterial volume flow, within the range of linear operation. Consideration has been given in flowmeter design to sensitivity, long term stability, and ease of operation.

ISSN:0034-6748    

DOI:10.1063/1.1135845

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A gas‐insulated gap switch with an unusually high standoff‐to‐trigger voltage ratio is described. Designed to standoff 500 kV when pressurized with SF6at 1.5 MPa (212 psi), the switch was triggered with as little as 19.3 kV across the gap by firing a shaped charge from one electrode toward the opposite electrode. Similar air‐insulated and SF6‐insulated gaps pressurized to 83 kPa (12 psi) and designed to standoff 50 and 150 kV, respectively, were triggered with 15.4 kV across the gap by firing an ordinary RP‐2 detonator from one electrode toward the other.

ISSN:0034-6748    

DOI:10.1063/1.1135839

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Two sensitive differential optical techniques useful for study of small reflectance or absorptive differences between two interfaces are described. The first scheme employs a mechanically chopped light beam impinging on an arrangement of symmetrical dual cells to allow detection of differential reflectances &Dgr;R/R<10−3. The second is a polarization‐modulated beam‐split laser version of the same method yielding approximately the same sensitivity. Representative measurements of voltage‐induced reflectance changes at liquid–solid interfaces using these methods are given.

ISSN:0034-6748    

DOI:10.1063/1.1135846

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

Shorted guard‐ring quartz gauges 12.7 mm in diameter and 3.2 mm thick were shock loaded by AL 6061 and copper impactors to stresses of 42 and 58 kilobars, respectively. Distortion of the current output was observed only at the 58 kilobar stress; at 42 kilobars the current profile conforms closely to the stress profile. Comparison of present data with previous studies demonstrates: (a) The present results lie on a smooth extension of an earlier calibration by Hayes and Gupta. (b) When only the current peak at the shock front is considered, the present data lie close to Graham’s curve for shunted gauges. An example of application of the shorted gauge at 40 kilobars to monitor a stress relaxing profile in LiF is also described.

ISSN:0034-6748    

DOI:10.1063/1.1135847

出版商:AIP    

年代:1979

数据来源: AIP

摘要:

A programmable pulse generator is described for NMR spectrometers. The pulse generator can operate either in a single cycle mode where up to 16 pulses can be given sequentially or, in the double cycle mode, a programmable number (⩽15) of these pulses can be repeatedntimes (n⩽99) before the rest of the 16 pulses are completed. The length of all these pulses can be programmed, manually or by computer. As an example the programming of the pulse generator for a multiple contact proton enhanced13C NMR experiment is given.

ISSN:0034-6748    

DOI:10.1063/1.1135848

出版商:AIP    

年代:1979

数据来源: AIP